If you do not use turrets are you getting attacked by 50+ size groups?

50+? I have more that 300 tribesmen coming, or 200+ in case of pirates. =p

It's survivable so far, but soon I'll reach the point where no matter how many enemies I kill, they just won't give up and force

*me* to run away instead.

Yeah I am aware of that, though. Even without turrets, raid strength depends on wealth, is it not? That means everything you have in your stockpiles, every single building, the walls, everything. So even without turrets, if one has a base size of the moon then the raid will be as big accordingly, or am I mistaken?

Yes, raid strength depends on wealth as well as your strength. Each colonist generates 26 threat points and each 1000 points of wealth generates 29 threat points. That is rather significant, my colony has 21 colonists right now and 150k wealth... which means I get 546 threat points from colonists and roughly 4350(!) from wealth, adding up to 4896 threat points total. That alone makes for a strong raid.

The most important thing determining raid strength seems to be time, though. There are two variables, shortTermFactor and longTermFactor. Both of these begin at 1 and are multiplied together to determine raid strength force. shortTermFactor starts increasing after 50 days of play and doubles every 80 days, longTermFactor begins after 120 days and doubles every 150 days. This becomes very significant for older colonies. Using my own 330 day old colony as an example again, the shortTermFactor is 4.5 and longTermFactor is 2.4, making the raids coming at me more than 10 times stronger

*(ie using 48960+ threat points!)*If you want to live just a little while longer, keep your wealth down (this is where wood walls come useful I guess). It won't save you for long, though.

I have gotten lucky on my last 2 raids that some friendly groups of 100+ showed up at the same time to do 90% of the work.

I had no idea this is even possible. The best I got was a group of 5 villagers coming to help with a siege of 200 raiders and they waited in the centre of my village instead of actually trying to help.